1. Field of the Invention
This invention relates to a method of forming oblique dot pattern in a tone production method by density of each element (described "element-density tone production method" hereinafter).
2. Prior Art
In order to record a gradation image utilizing a method employing two values comprising a dark value and a light value such as black and white, it is conventional practice to employ a method in which a half-tone is recorded by increasing or decreasing the number of recording dots which are recorded in each area of a predetermined matrix structure, this system being called an element-density tone production method. Various element-density tone production methods are: a density-pattern method which is constituted in such a manner that a plurality of dot areas are assigned to a picture element, a dither method in which one dot is assigned to a picture element, and the recording of each dot is determined by a threshold matrix with different thresholds, and a mixture of the above two methods. With an element-density tone production method, it is known that a better-quality image can be obtained if the recording dots are arranged in an aggregate manner having a certain regularity, rather than in such a way that they are arranged randomly. Various types having large aggregation units are known, such as a half-tone dot type and a vortex type. Known types in which aggregation units are dispersed are a dispersed type and the Bayer type. A type is selected in accordance with the characteristics of the image to be recorded.
When a color image is formed by the above-described element-density tone production method in which images of respective colors are superimposed, two major defects occur if the registration of the superimposed dots slips. One of them is the generation of moire fringes. If repetative patterns having the same or similar period are superimposed, fringe patterns with a long period, which are not present in the original image, are generated by any slight positional offset of the repetative patterns. Another defect occurs when dots of different colors are superimposed. The color produced when the dots are completely superimposed differs from the produced when the dots are offset, therefore color reproduction becomes unstable.
In printing, in order to overcome the above-described problems, the angle of each array of half-tone dots is changed in accordance with print color so that the period of moire fringes is restricted to a small value in order to make the moire fringes unnoticeable. Furthermore, the degree of superimpose of each colors dots is arranged to successively change in accordance with the location of each dot, keeping the degree of superpositioning constant as a whole.
Thanks to employment of the above-described means, if the position at which each color image is formed slips, the state of the printed image does not significantly change. Such means is a key technology for commercial color printing.
However, in a device which records an image based on information which has been converted into an electrical signal, the recording image dots are usually recorded at specific positions arranged in the vertical and horizontal directions of the recording paper because of restrictions imposed by the recording mechanism or recording head. Therefore, since they cannot be arranged in an oblique manner, as can be conducted in printing, the above-described problems cannot be overcome.
In the above-described element-density tone production method, since the dot recording is performed in such a manner that recording dots are aggregated in a matrix area having a predetermined number of dot recording positions, an attempt can be made to obtain the same effect as that obtained in printing by arranging half-tone dots in an oblique manner, by arranging central positions of the aggregated dots in an oblique manner.
For example, pages 15 to 20 in the proceedings of the Second Non-Impact Printing Technology Symposium sponsored by the Electrophotographic Society gives a method of obtaining an oblique arrangement of aggregated dots, FIG. 1A illustrating the method. In this figure, a.times.a squares surrounded by solid lines indicate matrix areas for element-density tone production, each constituted by an a.times.a dot areas. The positions of the dots in the matrix areas are assigned to gradation steps, the dots being dots to be recorded. The system is so constituted that one matrix pattern is assigned to one gradation step. In order to arrange the recording dots on an oblique line, the neighbouring matrix areas are arranged in such a manner that they are offset by b in both the longitudinal and lateral directions, creating c which are not assigned to any matrix areas. Therefore, when these areas c are dot-recorded, the formation of the recording dots is arithmetically interpolated in accordance with the state of the gradients of the neighbouring matrix areas. The angle of the aggregated dots can be changed by changing the value of b. Therefore, by changing the value of b in accordance with the colors to be superimposed, various oblique angles can be obtained.
FIG. 1B illustrates the state of superposition of the matrix areas when b=0, and that when b=a/4. The solid lines show the former case while the dashed lines show the latter case. This arrangement ensures that the matrix areas are so constituted that 16 ways of superpositioning repeat periodically. Therefore, even if the recorded position of the colors are offset, substantially no change in averaged superposition ratio occurs, and the period of moire fringes is significantly shortened making the moire fringes inconspicuous.
Although the conventional method shown in FIGS. 1A and 1B in which the aggregated dots are arranged in an oblique manner, is significantly effective in stabilizing the recorded image, the signal processing circuit for realizing the oblique arrangement of the aggretaged recorded dots is too complicated. That is, the factors which are needed to realize signal processing are as follows:
(1) Original recording image information is information that is arranged in the longitudinal and lateral directions of the recording sheet, as shown by the solid lines of FIG. 1B. If the original recording image information is converted by the density pattern method into the oblique arrangement shown by the dashed lines, the gradation step of each dashed-line matrix area must be assigned by calculation from the gradation steps of the solid-line matrix areas in the areas in which the dashed lines and the solid lines do not coincide. If the dither method is used, a dither pattern which has been previously arranged in an oblique manner and has a large size may be prepared, but, in this case, the matrix size of the dither pattern becomes extremely large.
(2) With the density pattern method, after the density of the obliquely-arranged matrix area has been determined, the matrix pattern of each assigned gradation step is read from a look-up table. After this matrix pattern has been edited on a screen to arrange the matrix pattern in an oblique manner, recording lines are read out along the lateral direction of the screen to suit the recording system, for the purpose of generating a raster signal.
(3) The recording dots for the empty areas c which are not assigned to any matrix areas must be arithmetically interpolated, therefore, these areas can easily cause errors in recording.
As described above, when the oblique arrangement method shown in FIGS. 1A and 1B is applied to the density pattern method, large-scale calculations must be performed at high speed. On the other hand, with the dither method, the complication of the calculations can be slightly reduced, but the size of the dither matrices becomes too large. In both of the above two methods, error regions c are inevitably formed in accordance with the conversion.